In present medical technology, delivering the drug to a lesion zone without passing the metabolism of the digestive system and the liver to maintain the concentration of the drug in the blood is a concerned research subject. However, it is difficult to deliver the drug to the lesion zone directly.
For example, the direct delivery of drugs to the central nervous system would make the resulting interactions highly target-specific and thereby dramatically improve the therapeutic effects and reduce possible side effect. However, it is difficult to delivery many potent therapeutic agents to the brain due to the presence of the blood-brain barrier, which is a specialized system of capillary endothelial cells that protects the brain from harmful substances. Although many methods have been developed to overcome the blood-brain barrier impermeability when delivering drugs, such as increasing their liquid solubility, or by the using vectors such as amino acids for carriers, none has been applied clinically.
Furthermore, the concentration of chemotherapeutics required to achieve clinically effective cytotoxicity in tumors is limited by the associated tissue toxicity and by physiologic barriers that prevent the delivery of drug to the tumor. Liposome-based drug-delivery systems have been designed to elevate tumor drug levels while limiting systemic drug exposure. It is thought that targeted delivery of liposomes encapsulating cytotoxic drugs should increase the accumulation and retention of drugs at the tumor site. The employment of liposomal chemistry, such as liposomes conjugated to antibodies or targeting ligands, can optimize and enhance the local delivery and better drug cell internalization compared with the free drug.
The therapeutic effect of a drug can be effectively increased when tissue-specific delivery of the drug is coupled to targeted biomarkers that may be expressed in certain disease conditions. This may also result in enhanced drug deposition while limiting systemic drug exposure. However, because each disease condition displays a different subset of biomarkers, and because each individual persons' biomarkers associated with a certain disease may be expressed at varying levels, the effects of targeted drugs may vary and sometime are less effective than expected.